This invention pertains generally to the formation of silicon-nitrogen compounds in integrated circuits, and more particularly to a lower temperature process for forming thick, thermally grown, silicon-nitrogen dielectrics.
Semiconductors are widely used in integrated circuits for electronic devices such as computers and televisions. These integrated circuits typically combine many transistors on a single crystal silicon chip to perform complex functions and store data. Semiconductor and electronics manufacturers, as well as end users, desire integrated circuits that can accomplish more functions in less time in a smaller package while consuming less power.
Most semiconductor memories use an array of tiny capacitors to store data. One approach to expanding the capacity of a memory chip is to shrink the area of each capacitor. However, everything else being equal, a smaller area capacitor stores less charge, thereby making it more difficult to integrate into a useful memory device. One approach to shrinking the capacitor area is to change to a storage dielectric material with a higher permittivity. Silicon nitride is one material that has a higher permittivity than the most conventional dielectric, silicon dioxide.
In another, related area, one concern is the thickness of the gate dielectric used in conventional CMOS circuits. The current drive in a CMOS transistor is directly proportional to the gate capacitance. Since capacitance scales inversely with thickness, higher current drive requires continual reductions in thickness for conventional dielectrics. Present technology uses SiO2 based films with thicknesses near 5 nm. However projections suggest the need for 2 nm films for future small geometry devices. SiO2 gate dielectrics in this thickness regime pose considerable challenges from a manufacturing perspective. In general, the increase in capacitance density (C/A) required for increasing current drive can be accomplished either by decreasing the dielectric thickness t or by increasing the dielectric permittivity xcex5 of the material. Thus, as with storage dielectrics, it is again desirable to change to a material with a higher permittivity, such as silicon nitride. Due to its permittivity, a 3.6 nm silicon nitride film can provide the same capacitance density as a 2 nm SiO2 film, while a 9 nm nitride film provides an equivalent oxide thickness of about 5 nm.
Another reason for using silicon nitride as a gate dielectric is its effectiveness as a diffusion barrier for boron and other dopant species. This barrier property allows a silicon nitride gate dielectric to limit dopant depletion from polysilicon gates.
Integrated circuit manufacturers have used chemically vapor deposited (CVD) silicon nitride as oxidation and diffusion masks for years. However, CVD (or deposited) silicon nitride typically does not have good enough electrical properties, such as breakdown voltage, for use as a gate or memory dielectric.
An alternate approach for forming silicon nitride is direct nitridation of a silicon surface. This process forms a compound often referred to as thermally grown or thermal silicon nitride. In general, thermal silicon nitrides often have electrical properties that are better than typical deposited nitrides. This difference is especially significant, when comparing nitrides formed with repeatable processes used in high volume production.
Until now, the processes for forming silicon nitride have not been suitable for forming thick, thermally grown, silicon nitride layers in production micron and submicron circuits. U.S. Pat. No. 4,277,320 to Beguwala, et al. describes some shortcomings of using earlier silicon nitride methods to form gate dielectrics. However, the ""320 patent describes a method that uses a 975 degree C. substrate to form a thermal silicon nitride.
We have known that we could form a very thin, high quality, thermal silicon nitride film by exposing a clean silicon substrate to a reactive nitrogen atmosphere at temperatures above 426 degrees C. However, this process yields films that have a self-limiting thickness of about 5 xc3x85 at 426 degrees C. Raising the substrate temperature increases this thickness somewhat, but the self-limiting thickness is still about 15 xc3x85 at 800 degrees C. In fact, some artisans have taught that high quality, thermal nitride films, with a thickness of 4.5 xc3x85, typically required temperatures near 1150 degrees C. Since most micron and submicron integrated circuits have limited thermal budgets, it is desirable to avoid steps that require high temperatures. Thus, we developed a method to form useful thicknesses of silicon nitride at temperatures below 900 degrees C., which can also be practiced below 900, 800, or even 500 degrees C.
A method for forming a thermal silicon nitride on a semiconductor substrate is disclosed. This method includes providing a partially completed integrated circuit with an exposed silicon surface; exposing the silicon surface to a first atmosphere including nitrogen, wherein the integrated circuit surface first temperature is between 426 and 700 degrees C., thereby forming an original layer of thermal silicon nitride, the silicon nitride layer""s thickness substantially determined by the silicon surface""s temperature; determining a planned integrated circuit surface temperature for a second silicon nitride layer formation, the planned temperature between 426 and 700 degrees C., thereby substantially determining the second silicon nitride layer""s potential thickness; depositing a layer of silicon on the original layer of silicon nitride to form a second silicon layer, the second silicon layer having a thickness no greater than the second silicon nitride layer""s potential thickness; exposing the second silicon layer to a second atmosphere including nitrogen, wherein the-integrated circuit surface second temperature is the planned temperature, thereby forming a second layer of thermal silicon nitride extending to the original layer of thermal silicon nitride and creating a combined layer of thermal silicon nitride. In some embodiments, wherein the first and second atmospheres include ammonia. In some embodiments, the first temperature is below 600 degrees C., and may be above 500 degrees C.
In another method, the method includes providing a semiconductor substrate with an exposed silicon surface, wherein the semiconductor substrate temperature is between 426xc2x0 C. and 900xc2x0 C.; exposing the silicon surface to a first atmosphere including a nitrogen source and a silicon source, under conditions where reactions in the atmosphere are generally avoided; wherein the silicon source deposits silicon on the exposed surface at a silicon growth rate and the nitrogen source reacts with the silicon on the integrated circuit surface to form thermal silicon nitride, wherein the silicon nitride reaction rate is limited by the availability of unreacted silicon on the exposed surface. In this method, two potential nitrogen sources are atomic nitrogen and ammonia. In some embodiments, the pressure of the first atmosphere is less than 10xe2x88x926 Torr, and may be above 10xe2x88x929 Torr.